8.9 Coupled Models - Dependence on Resolution

The importance of numerical aspects of climate models continues to be well
recognised and new numerical techniques are beginning to be tested for use in
climate simulation. However, there has been very little systematic investigation
of the impact of improved numerics for climate simulation and many important
questions remain unanswered. The degree of interaction between horizontal and
vertical resolution in climate models and the interaction of physical parametrizations
at differing resolutions has made it extremely difficult to make general statements
about the convergence of model solutions and hence the optimum resolution that
should be used. An important question regarding the adequacy of resolution is
deciding whether the information produced at finer scales at higher resolution
feeds back on the larger scales or do the finer scales simply add to local effects
(Williamson, 1999). Insufficient systematic work has been done with coupled
models to answer this question. As well as improving numerical accuracy in advection,
improved horizontal resolution can also improve the representation of the lower
boundary of a model (the mountains) and the land-sea mask; this may improve
the regional climate of a model but little systematic work has been carried
out to assess this aspect.

8.9.1 Resolution in Atmospheric Models

A series of experiments that explores convergence characteristics has been
conducted with the NCAR Community Climate Model (CCM) by Williamson (1999).
In these experiments the grid and scale of the physical parametrizations was
held fixed while the horizontal resolution of the dynamical core was increased.
As the dynamical resolution was increased, but the parametrization resolution
held fixed, the local Hadley circulation in the dual-resolution model simulations
converged to a state close to that produced by a standard model at the fixed
parametrization resolution. The mid-latitude transient aspects did not converge
with increasing resolution when the scale of the physics was held fixed. Williamson
(1999) concludes that the physical parametrizations used in climate models should
explicitly take into account the scale of the grid on which it is applied. That
does not seem to be common in parametrizations for global climate models today.

Pope et al. (1999) have also illustrated the positive impact of increased horizontal
resolution on the climate of HADAM3. A number of systematic errors evident at
low resolution are reduced as horizontal resolution is increased from 300 to
100 km. Improvements are considered to be mainly associated with better representation
of storms. It is apparent that, for some models at least, neither the regional
aspects of a climate simulation nor the processes that produce them converge
over the range of horizontal resolutions commonly used (e.g., Déqué
and Piedelievre, 1995; Stephenson and Royer, 1995; Williamson et al., 1995;
Stephenson et al., 1998). As part of a European project (High Resolution Ten-Year
Climate Simulations, HIRETYCS, 1998), it was found that increases in horizontal
resolution did not produce systematic improvements in model simulations and
any improvements found were of modest amplitude.

The need for consistency between horizontal and vertical resolution in atmospheric
models was first outlined by Lindzen and Fox-Rabinovitz (1989) but little systematic
study has been followed. Experiments with the NCAR CCM3 showed that increased
vertical resolution (up to 26 levels) above the standard 18 levels typical of
the modest vertical resolutions of climate models is beneficial to the simulations
(Williamson et al., 1998). Pope et al. (2000) also considered the impact of
increased (up to 30 levels) vertical resolution on simulations with HADAM3.
In both cases a number of improvements were noted due mostly to the improved
representation of the tropopause as the resolution was increased. However, Bossuet
et al. (1998) reached a somewhat different conclusion when they increased the
vertical resolution in the ARPEGE model; they concluded that increasing vertical
resolution produced little impact on the simulated mean climate of their model.
They also found that the physical parametrizations they employed were resolution
independent. Increased vertical resolution in the upper troposphere and stratosphere
has generally reduced model systematic errors in that region (Pawson et al.,
2000).

Enhanced regional resolution within an AGCM is possible through the global
variable-resolution stretched-grid approach that has been further developed
since the SAR (e.g., Dèquè and Piedelievre, 1995; Fox-Rabinovitz
et al., 1997); this is discussed in more detail in Chapter
10.

8.9.2 Resolution in Ocean Models

A number of important oceanic processes are not resolved by the current generation
of coupled models, e.g., boundary currents, mesoscale eddy fluxes, sill through
flows. Two model studies show an explicit dependence of ocean heat transport
on resolution, ranging between 4° and 0.1° (Fanning and Weaver, 1997a;
Bryan and Smith, 1998). However, this dependence appears to be much weaker when
more advanced sub-grid scale mixing parametrizations are used, at least at resolutions
of 0.4° or less (Gent et al., 1999). As previously noted, a number of recent
non-flux adjusted models produce acceptable large-scale heat transports. The
need for ocean resolution finer than 1° is a matter of continuing scientific
debate.

Some ocean models have been configured with increased horizontal resolution
(usually specifically in the meridional direction) in the tropics in order to
provide a better numerical framework to handle tropical ocean dynamics. Unfortunately
at this time, there has been little systematic intercomparison of such model
configurations.

8.9.3 Summary

The lack of carefully designed systematic intercomparison experiments exploring
impacts of resolution is restricting our ability to draw firm conclusions. However,
while the horizontal resolution of 2.5° (T42) or better in the atmospheric
component of many coupled models is probably adequate to resolve most important
features, the typical vertical resolution of around 20 levels is probably too
low, particularly in the atmospheric boundary layer and near the tropopause.
The potential exists for spurious numerical dispersion, when combined with errors
in parametrizations and incompletely modelled processes, to produce erroneous
entropy sources. This suggests that further careful investigation of model numerics
is required as part of a continuing overall programme of model improvement.
The vertical resolution required in the ocean component is still a matter of
judgement and tends to be governed by available computing resources. There is
still considerable debate on the adequacy of the horizontal resolution in the
ocean component of coupled models and it is suggested that some results (those
that are reliant on meridional heat transport) from coupled models with coarse
(>1°) resolution ocean components should be treated cautiously.